In this commit, we add License for XiangShan project. 

* Set EMU_FORKWAIT=1 will enable fork-wait function and enable verilator
trace by default.

* Fix the zombie child process problem.
* All information of standard ouput can be redirected to log.

* Every 5 seconds fork a child process.

*Move the fork-wait code at the end of execute while loop.
*Use list in STL instead of array to represent pid list.

-> Leave a problem need to be solve, which is checkpoint is the error
scene.

* emu: add --no-perf-counter option

Now perf counter result print will no longer be controlled by
--log-begin / --log-end

* emu: add --force-dump-result option

This option will override log_end to -1 when simulation finishs.
--no-perf-counter option is removed.

* difftest: revise coreid assignment

* nemuproxy: compatible for smp difftest

* difftest: fix goldenMem initialization problem

* difftest: goldenMem update works

* difftest: api compatible for modified nemu

* difftest: support load check for smp difftest
* verification is required later

* misc: remove unused xstrap wiring

* Remove unused code

* difftest: add latch for difftest-loadevent

* misc: update inclusivecache

* difftest: reset resp for sbuffer & atomic-unit to avoid duplicate update
of goldenMem

* difftest: dump coreid when difftest failed

* difftest: dump corresponding memory of another core when smp difftest failed
* Only works for dual-core

* difftest: fix interrupt handler

* difftest: cleanup code

* roq: remove legacy signal for difftest

* Emu: stop sim if assertion is triggered

* Difftest: fix record_inst wdata width

* Difftest: fix xs_assert

* fix assert bug in L3
Co-authored-by: NLinJiawei <linjiawei20s@ict.ac.cn>

* this can switch to different reference design without re-compile emu

* difftest: use DPI-C to refactor difftest

In this commit, difftest is refactored with DPI-C calls.
There're a few reasons:
(1) From Verilator's manual, DPI-C calls should be more efficient than accessing from dut_ptr.
(2) DPI-C is cross-platform (Verilator, VCS, ...)
(3) difftest APIs are splited from emu.cpp to possibly support more backend platforms
(NEMU, Spike, ...)

The performance at this commit is quite slower than the original emu.
Performance issues will be fixed later.

* [WIP] SimTop: try to use 'XSTop' as soc

* CircularQueuePtr: ues F-bounded polymorphis instead implict helper

* Refactor parameters & Clean up code

* difftest: support basic difftest

* Support diffetst in new sim top

* Difftest; convert recode fmt to ieee754 when comparing fp regs

* Difftest: pass sign-ext pc to dpic functions && fix exception pc

* Debug: add int/exc inst wb to debug queue

* Difftest: pass sign-ext pc to dpic functions && fix exception pc

* Difftest: fix naive commit num limit
Co-authored-by: NYinan Xu <xuyinan1997@gmail.com>
Co-authored-by: NWilliam Wang <zeweiwang@outlook.com>